Low‐latitude total electron content enhancement at low geomagnetic activity observed over Japan

Abstract

Numerous total electron content (TEC) values derived from GPS signals are averaged within 1.5° × 1.5° cells in a 1‐hour time frame, and the relative deviations of these average values from corresponding monthly medians are used to produce latitude‐time plots over Japan. The paper analyzes the appearance and development of enhancements of TEC of equatorial origin (ETEs), occurring outside initial and main phases of geomagnetic storms. ETE structures appear mainly as single‐crest structures in the evening hours local time, with TEC peak around 1900 LT. TEC usually decreases with latitude, and the structures disappear below 40°N. In some cases the TEC peak is found above the plot boundary of 24°N, as depletions toward the equator are also observed. The observed enhanced structures are linked to the well‐known evening prereversal enhancement of ion drift in the equatorial F region. Double‐crest ETEs are also observed, with the second peak occurring in early morning hours. Most of the ETE events appear in periods of low geomagnetic activity, 1–3 days after the main phase of the storms. In some cases the time of rising of ETE structures coincides with the increase of interplanetary electric field (IEF), a fact showing the importance of directly penetrating electric field in formation of ETEs. Often, ETEs appear repeatedly in 2 or 3 consecutive days. It is supposed that planetary atmospheric waves are responsible for this phenomenon. Most of the observed features of ETEs can be explained by the published results of simulations of the coupled thermosphere‐ionosphere‐plasmasphere (CTIP) model and the thermosphere/ionosphere/electrodynamic general circulation model (TIEGCM). It is suggested that ETE structures are produced mainly by a disturbance winds dynamo electric field, built up after the main phase of the storms. Some ETE events, appearing at the end of a prolonged period of low geomagnetic activity, can be linked to directly penetrating IEF in equatorial ionosphere. The intensity of these electric fields is much smaller than is needed to explain the observations.